Project 3 is designed to identify and assess factors related to the Giardia lamblia parasite and to the host-parasite interaction which may account for the variability in duration and clinical spectrum of G. lamblia infection, as documented in our previous studies and the current ICIDR. In collaboration with Projects 1 and 2 and the Microbiology Core, G. lamblia will be isolated and cultured in vitro from Bedouin infants enrolled in the prospective cohort studies as well as from symptomatic giardiasis patients. These isolates will be characterized and compared by isoenzymes analysis and by karyotype analysis using pulsed field gel gradient electrophoresis. Isolate markers associated with presence or absence of symptomatology and with short-term, intermittent or long-term infection will be sought. The host-parasite interaction will be studied using in vitro models. In order to identify isolates with potential virulence properties, the cytopathic effect of G. lamblia trophozoites on monolayers of the small intestinal-like epithelial cell line Caco-2 will be compared using isolates from symptomatic and asymptomatic subjects. In collaboration with Project 4 and the Serology Core, the basis for persistent and recurrent G. lamblia infection will be investigated. The humoral and cell-mediated immune response to homologous and heterologous G. lamblia isolates will be assessed in in vitro assays of anti-Giardia titers and giardiacidal activity of serum and leukocyte subsets from symptomatic and asymptomatic subjects with short-term versus recurrent or persistent infection. A diagnostic test based on polymerase chain reaction (PCR) technology will be developed for detection of G. lamblias in stool samples and water supplies and will be implanted in the cohort (Project 1) and intervention (Project 2) studies. These techniques will also be applied to the typing of G. lamblia directly in stools, without the need for in vitro culture, by PCR ribotyping. These studies will elucidate mechanisms for the variable patterns of infection and pathogenicity of this ubiquitous enteric parasite.